An organic compound absorbs light, thereby producing an excited state. The organic compound undergoes this excited state; accordingly, luminescence or various reactions (such as photochemical reactions) may occur. Therefore, various applications of the organic compounds have been made.
A light-emitting element including an organic compound as a light-emitting substance has a simple structure in which a light-emitting layer containing the organic compound that is the light-emitting substance is provided between electrodes. This light-emitting element has attracted attention as a next-generation flat panel display element in terms of its characteristics such as thinness, lightness, high-speed response, and DC drive at low voltage. Further, a display including this light-emitting element is superior in contrast, image quality, and wide viewing angle.
The light-emitting element including an organic compound as a light-emitting substance has a mechanism of light emission, which is a carrier injection type: a voltage is applied between the electrodes where the light-emitting layer is interposed, electrons and holes injected from the electrodes recombine to produce an excited state of the light-emitting substance, and then light is emitted when the light-emitting substance returns to a ground state from the excited state. As in the case of the photoexcitation, types of the excited state of organic compounds include a singlet excited state (S*) and a triplet excited state (T*). Furthermore, it is thought that the ratio of S* to T* in a light-emitting element is statistically 1:3.
At room temperature, a compound that enables luminescence to occur with energy of a singlet excited state (hereinafter, referred to as a fluorescent compound) exhibits only luminescence from the singlet excited state (fluorescence), not luminescence from the triplet excited state (phosphorescence). Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) of a light-emitting element including a fluorescent compound is assumed to have a theoretical limit of 25% based on the ratio, S*:T*=1:3.
On the other hand, with a light-emitting element including a compound that enables luminescence to occur with energy of a triplet excited state (hereinafter, referred to as a phosphorescent compound), the internal quantum efficiency can be improved to 75 to 100% in theory; namely, the emission efficiency can be 3 to 4 times as high as that of a light-emitting element including a fluorescent compound. Therefore, light-emitting elements including phosphorescent compounds have been actively developed in recent years in order to achieve a highly-efficient light-emitting element, (e.g., see Non-patent Document 1). An organometallic complex that contains iridium or the like as a central metal is particularly attracting attention as a phosphorescent compound because of its high phosphorescence quantum efficiency.